<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        body{
            height: 100vh;
            display: flex;
            flex-direction: column;
            justify-content: space-between;
        }
        li{
            list-style: none;
        }
        main{
            flex: 1;
            background-color: #ddd;
        }
        footer{
            height: 50px;
            background-color: #ff6700;
            display: flex;
            border-top: 1px solid #ccc;
        }
        footer section{
            flex: 1;
            line-height: 50px;
            text-align: center; 
            color: #fff;
            font-size: 25px;
            display: flex;
            flex-direction: column-reverse;
        }
        footer section:nth-child(n+2) {
            border: 1px solid #ccc;
        }
        footer section:last-child{
            border-right: none;
        }
        footer section h4{
            flex: 0 0 50px;
            text-align: center;
            justify-content: center;
            display: flex;
            cursor: pointer;
        }
        footer section ul{
           display: flex;
           flex-direction: column;
           border: 1px solid #ccc;
           color: black;
           text-align: center;
           border-collapse: collapse;
           margin-bottom: 5px;
           /* display: none; */
        }
        footer section ul li {
            border: 1px solid #ccc;
            cursor: pointer;
        }
        footer section ul li:last-child{
            border-bottom: none;
        }
        footer section h4:hover .btn{
            display: block !important;
        }
    </style>
</head>
<body>
    <main></main>
    <footer>
        <section>
            <h4>教程</h4>
            <ul class="btn">
                <li>css</li>
                <li>php</li>
            </ul>
        </section>
        <section>2</section>
        <section>3</section>
    </footer>
</body>
</html>